Image suppression system



Dec. 1, 1936. R. B. ALBRIGHT 2,062,956

IMAGE SUPPRESSION SYSTEM Filed Nov. 5, 1955 I 5;; xvi; 6

Patented Dec. 1, 1936 UNITED STATES PATENT OFFICE IMAGE SUPPRESSIONSYSTEM Application November 5, 1935, Serial No. 48,407

5 Claims.

This invention relates to novel signal transfer means for radioreceivers and like systems, wherein it is desired to transfer signals ofcertain frequency and to reject signals of certain other frequencies.More particularly, the invention relates to signal transfer means of thestated character for transferring desired signals from a signal source,such as an antenna, to an amplifier stage while suppressing orattenuating undesired signals emanating from the said source.

One object of the invention, therefore, is to provide novel signaltransfer means adapted to transfer desired signals and to rejectundesired signals. 7

Another object of the invention is to provide signal transfer meanswhich, in addition to rejecting undesired signals, is adapted totransfer desired signals with substantial gain.

The invention is particularly adapted for use in a superheterodyne radioreceiver to couple the antenna to a tuned radio frequency stage and,when thus employed, the invention is adapted to suppress or reject theimage signals commonly encountered in such a system. For the purpose ofdisclosure, therefore, the invention will be described with particularreference to this adaptation but it will be understood that theinvention is not thus limited in scope.

In the accompanying drawing, the single figure is a diagrammaticillustration of a preferred form of the invention as applied to asuperheterodyne radio receiver.

As is now well known, in a superheterodyne receiver, the desired signalis modulated by a signal from a local oscillator producing signalshaving frequencies equal to the sum and difference of the desired signalfrequency and the oscillator frequency. The difference frequency signal,having the intermediate frequency, is selected and implified. Theoscillator frequency is thus the desired signal frequency plus theintermediate frequency. Likewise undesired signals known as imagesignals, whose frequency is equal to the oscillator frequency plus theintermediate frequency, or twice the oscillator frequency minus theintermediate frequency, will, when modulated with the oscillator signal,produce undesired signals of intermediate frequency. For example, if thedesired signal frequency band is from 150 to 300 k. c., i. e., the longwave band, and the intermediate frequency is 450 k. c., the image signalfrequency band will extend from 750 to 1200 k. c.

The present invention, when applied to a superheterodyne radio receiver,is adapted to suppress or reject undesired or image signals, such asthose above mentioned. For the purpose of the present specification andappended claims, an image signal may be defined as a signal other thanthe one it is desired to receive which will heterodyne with the localoscillator signal or a harmonic thereof to produce a signal, at leastone component of which is of the intermediate frequency.

Referring now to the single figure of the drawing, the antenna A may beconnected through an inductance L1 to one extremity of the primarywinding P of a tuned transformer T, the other extremity of the primaryWinding being connected to ground. The secondary winding S of thetransformer may be shunted by a variable condenser C which serves totune the secondary to the signal which it is desired to receive, as wellknown in the art. One extremity of the secondary winding may beconnected to the control element of a space discharge device V of aradio frequency transfer stage, while the other extremity of thesecondary winding may be connected to any suitable source of biasingpotential such as is commonly employed in the art. The transfer stagemay be part of any suitable means for'utilizing the desired signals andin the specific adaptation of the invention to a superheterodynereceiver, this transfer stage will form a part of the conventionalsuperheterodyne radio receiver. The primary of the transformer T may beuntuned and may be self-resonant at some frequency below the range offrequencies which it is desired to receive, as well known in the art.

In accordance with the present invention, there is provided a capacitiveelement C1 shunted about the primary of transformer T. It has been foundthat the provision of this element in cooperative relation with theother elements of the system results in effective attenuation of imagesignals and has other desirable features, as set forth hereinafter.Preferably, there is also provided a trap circuit L2C2 connecting theantenna to ground, which circuit is tuned to the intermediate frequencyand serves to reject signals of that frequency, as more fully explainedlater.

The invention may be clearly understood by a consideration of the designand functioning of the elements of the disclosed system in a particularcase. Assuming, for example, that it is desired to receive signalswithin the frequency range of 150 to 300 k. c. as above mentioned, thisbeing the standard long wave band, and assuming further that thesesignals are to be received with a superheterodyne radio receiver havingan intermediate frequency of 450 k. c., then it is apparent that imagesignals having the frequencies above mentioned will be encountered andthe system should be designed or adapted accordingly. The inductance L1may have such distributed capacity that it is self-resonant at afrequency of about 750 k. 0. Likewise, the primary winding of thetransformer T, due to its distributed capacity, may be self-resonant ata frequency of about 85 k. 0. These inherent capacities are indicated onthe drawing by brokenline representations, as is also the inherentcapacity between the windings of the transformer T. It will be seen thatthe transformer constitutes a signal transfer device having a certainamount of inherent capacitive transfer impedance as well as inductivetransfer impedance, and the directions of winding of the primary andsecondary may be designed so that transferred signals due to the one addwith signals transferred by the other. In the preferred form of theinvention, it is also preferable that the inductance L1, the capacitanceC1, and the primary of the transformer, when considered as a unit, havean over-all resonant frequency just above the long-wave frequency bandabove mentioned but below the intermediate frequency. For example, thisresonant frequency may be 335 k. c.

Considering now the reception of desired signals within the long-Wavefrequency range of 150 to 300 k. c., the trap circuit L2C2 will appearto such signals as a capacitive reactance having a very high impedance.The self-resonant inductance L1 will appear to such signals as aninductance, while the self-resonant primary winding of the transformerwill appear as a capacitive reactance. Therefore, the primary of thetransformer and the shunt capacitance C1 may be considered togethersince they will appear to the desired signals as a capacitive reactance.These characteristics of the elements in question will be obvious fromthe various frequencies involved.

It will be seen that the voltage across the inductance L1 will beapproximately 180 out of phase with the voltage across the capacitanceC1 and, therefore, the voltage appearing across the primary of thetransformer or signal transfer device will be somewhat greater than theinput voltage supplied by the intenna. In other words, the voltageapplied to the primary of the transformer will be equal to the sum ofthe input voltage and the voltage across the inductance L1. Thus, thedesired signals are transferred with a certain amount of gain.Furthermore, since the elements in question, considered as a unit, areresonant at a frequency of about 335 k. c., the gain will be greater atthe higher end of the longwave frequency band due to the upward slope ofthe response characteristic in the vicinity of the resonance peak.

At the desired signal frequencies in question, the transfer impedancebetween the primary and secondary of the transformer will be largelyinductive and will comprise substantially the mutual inductance betweenthe two windings. Furthermore, the impedance of the primary winding willbe small compared with the impedance of condenser C1. Thus, it will beseen that substantially all of the signal current flowing throughinductance L1 will flow through the primary winding of the transformerand the signal transfer to the secondary winding will be large.

Considering now the reception of signals having the intermediatefrequency or having frequencies in the neighborhood of the intermediatefrequency, the tuned trap circuit L202 will, of course, present very lowimpedance to such signals but the impedance of the remainder of thecircuit to such signals will be relatively high, as above mentioned.Thus, the antenna is substantially short circuited by the intermediatefrequency trap L202 and there will be substantially no signal transferthrough the device to the receiver.

Considering now the reception of signals in the image signal frequencyrange, that is, the frequency range from 750 to 1200 k. c., whichsignals it is desired to attenuate as much as possible, it will be seenthat the trap circuit L2C2 will appear to these signals as an inductancehaving very high impedance. The inductance L1, which is self-resonant ata frequency in the lowest part of the image signal frequency band, willappear to signals at that frequency as a very high impedance and willappear to signals of higher frequency as a capacitive reactance havinghigh impedance. It has been found further that the signal transferdevice T presents a transfer impedance which is largely capacitive atsuch frequencies and due to the capacitive coupling between the'primaryand secondary windings.

- It will be seen then that image signals from the antenna will passthrough the high capacitive impedance presented by the element L1 andwill then be transferred to ground through the condenser G1 which isshunted by the primary winding and by the circuit comprising thecapacitive signal transfer impedance and the tuning condenser of thesecondary circuit. Obviously, the smaller the reactance of the condenserC1, i. e., the larger the capacitance, the greater will be theattenuation of the image signals. It is further desirable that thetransfer impedance to image signals be as large as possible to increasefurther the image signal attenuation. For this reason it is necessary touse a separate condenser C1 rather than design the primary to have alarger distributed capacity. It is not practical to wind a transformerhaving sufiicient distributed capacity to replace C1 and even if itwere, the device would not function in the same manner as with theexternal condenser due to the presence of secondary resonance frequencypeaks which are to be found in the impedance characteristic above thefundamental resonant frequency of a coil having distributed capacity.

In order to obtain as great attenuation of the image signals aspossible, the condenser C1 should be made as large as possible but yetsufficiently small so that the unit comprising the inductance L1, thecondenser C1 and the primary winding is resonant at some frequency belowthe image frequency range and above the frequency range of the signalswhich it is desired to receive.

From the above description, it will be seen that the use of theinvention aids materially in suppressing or rejecting the undesiredsignals while transferring the desired signals with sufficient gain.Although the invention has been illustrated in preferred form and withrespect to a particular adaptation thereof, it will be understood thatit is not thus limited but is capable of modification within the scopeof the appended claims.

I claim:

1. In a superheterodyne radio receiver, an antenna, a signal transferdevice comprising a primary and a tuned secondary inductively coupledthereto, and having at least some capacitative transfer impedancetherebetween, an impedance element connected between said antenna andsaid transfer device, said impedance being inductive for signals of thedesired frequency and capacitive for signals of at least some imagesignal frequencies, and means for reducing image signals comprising aseparate capacitative impedance element in shunt relation with saidprimary.

2. In a superheterodyne radio receiver, an antenna, a signal transferdevice comprising a primary and a tuned secondary inductively coupledthereto, and having at least some capacitative transfer impedancetherebetween, an inductance element serially connected between saidantenna and said transfer device, said inductance element havingsufficient capacitive reactance associated therewith so that saidelement will appear as a capacitive reactance for signals of at leastsome image signal frequencies, and means for reducing image signalscomprising a separate capacitative impedance element in shunt relationwith said primary.

3. In a superheterodyne radio receiver, an antenna, a signal transferdevice comprising a primary and a tuned secondary inductively coupledthereto, and having at least some capacitative transfer impedancetherebetween, an inductance element serially connected between saidantenna and said transfer device, said inductance element beingself-resonant at a frequency at the lower portion of the image frequencyband of said receiver, and means for reducing image signals comprising aseparate capacitative impedance element in shunt relation with saidprimary.

4. In a superheterodyne radio receiver, an antenna, a signal transferdevice comprising a primary and a tuned secondary inductively coupledthereto, and having at least some capacitative transfer impedancetherebetween, an impedance element connected between said antenna andsaid transfer device, said impedance being inductive for signals of thedesired frequency and capacitive for signals of at least some imagesignal frequencies, a trap circuit tuned to the intermediate frequencyconnecting said antenna to ground, and means for reducing image signalscomprising a separate capacitative impedance element in shunt relationwith said primary.

5. In a superheterodyne radio receiver, an antenna, a signal transferdevice comprising a primary and a tuned secondary inductively coupledthereto, and having at least some capacitative transfer impedancetherebetween, an impedance element serially connected between saidantenna and said transfer device, said impedance being inductive forsignals of the desired frequency and capacitive for signals of at leastsome image signal frequencies, and means for reducing image signalscomprising a separate capacitative impedance element in shunt relationWith said primary, said device and said elements having an over-allresonant frequency above the frequency range of desired signals butbelow the intermediate frequency.

ROBERT B. ALBRIGHT.

